Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

The present invention relates to a supply circuit (101) comprising: a
bridge circuit (103), at least one resonant circuit (107) coupled to the
bridge circuit (103), the at least one resonant circuit (107) being
coupleable to a load circuit (109) comprising one or more loads (111), at
least one supply switching unit (129) coupled between the bridge circuit
(103) and an associated load circuit (109) for connecting and
disconnecting the load circuit (109) from the bridge circuit (103), and a
control unit (131) for controlling the at least one supply switching unit
(129) in synchronization with a resonant current (Ires) of the
resonant circuit (107) associated with said supply switching unit (129).

Claims:

1. Supply circuit comprising: a bridge circuit, at least one resonant
circuit coupled to the bridge circuit, the at least one resonant circuit
being coupleable to a load circuit comprising one or more loads, at least
one supply switching unit coupled between the bridge circuit and an
associated load circuit for connecting and disconnecting the load circuit
from the bridge circuit, and a control unit for controlling the at least
one supply switching unit in synchronization with a resonant current of
the resonant circuit associated with said supply switching unit.

2. Supply circuit as defined in claim 1, wherein a plurality of resonant
circuits is coupled to the bridge circuit and each resonant circuit being
coupleable to a load circuit.

4. Supply circuit as defined in claim 1, the bridge circuit comprising a
full bridge.

5. Supply circuit as defined in claim 1, wherein each resonant circuit is
provided with at least one supply switching unit.

6. Supply circuit as defined in claim 1, wherein the at least one supply
switching unit is connected in series to the one or more loads of the
load circuit.

7. Supply circuit as defined in claim 1, wherein the control unit is
adapted for providing a maximum switching frequency of the bridge
circuit, which is half of a resonant frequency of the resonant circuit.

8. Supply circuit as defined in claim 1, wherein the control unit is
adapted for providing a maximum switching frequency of the bridge
circuit, which is half of the lowest resonant frequency of a plurality of
resonant circuits.

9. Supply circuit as defined in claim 1, comprising three resonant
circuits, whereas the first of said resonant circuits being coupled to a
load circuit comprising at least one red LED and/or OLED, the second of
said resonant circuits being coupled to a load circuit comprising at
least one green LED and/or OLED, and the third of said resonant circuits
being coupled to a load circuit comprising at least one blue LED and/or
OLED.

10. Supply circuit as defined in claim 1, wherein the supply switching
unit is a transistor, in particular a MOSFET transistor.

11. Supply circuit as defined in claim 1, wherein the control unit is
adapted for switching on or off the supply switching unit during the
second negative half-wave of the resonant current.

12. Device comprising the supply circuit as defined in claim 1 and at
least one load circuit.

13. Device as defined in claim 12, wherein each load circuit being
coupleable to a resonant circuit comprises one or more LEDs and/or one or
more OLEDs.

14. Device as defined in claim 12, wherein at least one of the load
circuits comprising LED strings an anti-parallel configuration.

15. Device as defined in claim 12, wherein at least one photosensitive
component is provided interacting with at least one load circuit.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a supply circuit and to a device
comprising a supply circuit and at least one load circuit.

BACKGROUND OF THE INVENTION

[0002] Supply circuits, in particular switched mode power supplies, are
well known in the art. Such supply circuits are used to power consumer
and non-consumer products. An exemplary application is the powering of
light-emitting diodes (LEDs) and/or organic light-emitting diodes (OLEDs)
or other lighting systems, in particular LED strings used in professional
LED lighting systems, in particular lighting systems, where a precise
color control is required. Further applications are LED backlighting,
effect and accent illumination used for consumer products like "living
colors lamps" and furthermore color temperature adjustment in general
illumination applications as well as homogenizing the light output of
single LEDs of the multiple LED lamp with a single supply circuit. It
should be noted that in the following, the term "LED" is used as a
generic term for similar applications such as OLEDs or the like.

[0003] Supply circuits that are best suited and therefore are preferably
used for the above-named applications are in particular discontinuous
series resonant converter with a constant average current output, in the
following denoted as DSRC-I. The functionality of this type of converter
is commonly known, e.g., from WO 2007/148271 A2 or WO 01/45241 A1 and
shall therefore not be explained in detail. DSRC-I converters provide the
advantage of a constant average current output, furthermore, no current
sensing and no current control loop is required. Consequently, losses
caused by a current sensing are avoided, and the DSRC-I provides a
high-efficient, compact and easy design compared to other commonly known
supply circuits. Above all, it is open and short circuit proof.

[0004] The basic DSRC-I topology provides only one single output of the
supply circuit, in order to power a single load circuit, which is coupled
to the supply circuit. However, with regard to the application of the
supply circuit in general illumination applications, it is desirable to
not only have one but multiple outputs in order to power different load
circuits, e.g., comprising different colored LED strings such as red,
green and blue LED strings.

[0005] As an example, a "living colors lamp" needs to be able to emit
light of a huge number of colors generated by individually dimming and
mixing the emitted light of red, green and blue LEDs in a certain ratio.
Thus, by mixing the base colors red, green and blue (RGB), more than 16
million colors may be calculative generated. Also with regard to
backlighting of liquid crystal displays (LCDs), LEDs are increasingly
becoming the technology of choice. Thereby, the supply circuit is the key
to the quality of the backlight image. The driver i.e., the supply
circuit must be capable of providing a satisfactory dimming range that is
high brightness for daylight vision and low brightness for night vision.

[0006] The commonly used dimming method for a DSRC-I is to reduce the
switching frequency. However, this method is limited and hence does not
provide a satisfactory dimming range. Furthermore, reducing the switching
frequency will influence all outputs of a DSRC-I having multiple outputs.
Consequently, this method allows no individual dimming of single outputs
and thus, individual dimming for example of red, green and blue light and
consequently, does not permit color control.

[0007] There have been made attempts to realize individual dimming LED
strings. As an example, WO 2008/110978 describes a method, which uses
additional switches in the load circuit to bypass the load. However,
bypassing the load has harmful influences on the converter stability and
reduces the overall efficiency.

[0008] As can be seen from the above explanations, there is a need for
providing a supply circuit as well as a device comprising said supply
circuit permitting individual dimming of individual outputs of the
converter without negative influences on the converter stability and
having a compact and easy design.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a supply
circuit offering an individual full range dimming and high-efficiency
combined with a simple control.

[0010] In a first aspect of the present invention a supply circuit
comprising: [0011] a bridge circuit, [0012] at least one resonant
circuit coupled to the bridge circuit, the at least one resonant circuit
being coupleable to a load circuit comprising one or more loads, [0013]
at least one supply switching unit coupled between the bridge circuit and
an associated load circuit for connecting and disconnecting the load
circuit from the bridge circuit, and [0014] a control unit for
controlling the at least one supply switching unit in synchronization
with a resonant current of the resonant circuit associated with said
supply switching unit. The number of supply switching units preferably
corresponds to the number of resonant circuits or in other words, at
least one supply switching unit is assigned to each resonant circuit.

[0015] Hence, full range dimming is achieved by inserting just one supply
switching unit per load circuit in the supply circuit, in particular in
the at least one resonant circuit, which connects and disconnects the
load for a desired number of switching cycles of the bridge circuit by
means of a control unit. Further, this supply switching unit can be gated
very easily and a normal MOSFET is sufficient, as for this method no
bidirectional blocking switch is required. Additionally, the advantage to
interleave certain LED strings is offered. Furthermore, the above-named
advantages of the DSRC-I converter itself are still valid with this
modification. Consequently, there is still no current sensing and
controlling required. Furthermore, an individual full range dimming down
to zero is permitted, whereas a high-efficiency is provided at all
dimming levels. Altogether, this invention offers in particular the
advantage of controlling several different LED strings individually with
only one central converter.

[0016] In a further aspect of the present invention a supply circuit is
presented, wherein a plurality of resonant circuits is coupled to the
bridge circuit and each resonant circuit being coupleable to a load
circuit, each load circuit constituting an output of the supply circuit.
Preferably, per resonant circuit, one supply switching unit is provided,
whereas the supply switching unit in particular forms part of the
respective resonant circuit. Hence, each resonant circuit is preferably
provided with a supply switching unit.

[0017] In particular, the supply circuit is preferably provided with three
resonant circuits, whereas the first of said resonant circuits being
coupled to a load circuit comprising one or more red LEDs and/or OLEDs,
the second of said resonant circuits being coupled to a load circuit
comprising one or more green LEDs and/or OLEDs, and the third of said
resonant circuits being coupled to a load circuit comprising one or more
blue LEDs and/or OLEDs. With this configuration it is possible to control
the color of the light emitted by the respective device, as the different
colored LEDs of each load circuit may be controlled individually to
generate a desired color mixture.

[0018] In a further aspect of the present invention a supply circuit is
presented, wherein the bridge circuit is based on a half bridge. However,
it is also possible that the bridge circuit is based on a full bridge.

[0019] As mentioned above, dimming of a DSRC-I is basically realized by
reducing the switching frequency. This method is preferably used with the
full-bridge DSRC-I, because the peak value of the additional current in a
zero voltage switching (ZVS) snubber circuit, which will be explained in
more detail later on, remains constant if the switching frequency is
reduced. With the half-bridge configuration this current increases if the
switching frequency is reduced. Hence, a half-bridge DSRC-I offers no
good dimming solution, when dimming is performed by varying the switching
frequency. The present invention, however, may be carried out by using a
half bridge as well as a full bridge, whereas using a half bridge
configuration is even preferred, as only two switches are required for
realizing the bridge circuit and therewith an easy control of the
switches is provided.

[0020] In a further aspect of the present invention a supply circuit is
presented, wherein the supply switching unit is connected in series to
the one or more loads of the load circuit.

[0021] This provides the advantage that the load is not only bypassed,
which has harmful influences on the converter stability and leads to a
reduction of the overall efficiency. Rather, the load is simply
disconnected for a desired number of switching cycles from the supply
circuit. In this respect, the term "disconnected" means that current flow
from the load circuit back to the bridge circuit is interrupted by the
supply switching unit.

[0022] In a further aspect of the present invention a supply circuit is
presented, wherein the control unit is adapted for providing a maximum
switching frequency of the bridge circuit, which is half of a resonant
frequency of the resonant circuit. Furthermore, a control unit is
preferably adapted for providing a maximum switching frequency of the
bridge circuit, which is preferably half of the lowest resonant
frequency, if a plurality of resonant circuits are employed, whereas the
resonant circuits defining different resonant frequencies. The control
unit may be the control unit driving the supply switching unit, however,
it may also be a separate control unit only driving the bridge circuit.

[0023] In a further aspect of the present invention a supply circuit is
presented, wherein the control unit is adapted for switching on or off
the supply switching unit during the second negative half-wave of the
resonant current. This is advantageous as the resonant current commutates
on the body diode of the supply switching unit and stops after it reaches
zero. If the load is switched off, it is switched on again by a switching
on the respective supply switching unit, in particular the respective
MOSFET in the same time interval that is in the second negative half-wave
of the resonant current.

[0024] It is another object of the present invention to provide a device
comprising the supply circuit as defined in claim 1 and at least one load
circuit.

[0025] In a further aspect of the present invention a device is presented,
wherein each load circuit coupleable to a resonant circuit comprises one
or more LEDs and/or one or more OLEDs, each having a different color.
Hence, each load circuit, i.e., each output of the supply circuit may
comprise at least one LED/OLED of a specific color, in particular red,
green or blue. This provides the advantage of dimming each of these load
circuits and with this each of the different colored LEDs individually.

[0026] In a further aspect of the present invention a device is presented,
wherein at least one of the load circuits comprising LED strings having
an anti-parallel configuration. This provides the advantage that the load
may be operated by alternating current (AC).

[0027] In a further aspect of the present invention a device is presented,
wherein at least one photosensitive component is provided interacting
with at least one load circuit. This allows the easy detection of a
system fault or a damaged LED as well as calibrating the LEDs or
compensating aging effects. Above all, a perfect color control is
feasible with the photo sensitive component.

[0028] Consequently, a novel supply circuit and a corresponding device
comprising the supply circuit are presented for effectively dimming a
load circuit and in particular performing precise color control. The
solution for these requirements is described in this invention.

[0029] These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiment described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In the following drawings

[0031] FIG. 1 illustrates a block diagram of a supply circuit known in the
art coupled to a load circuit;

[0032]FIG. 2 illustrates a block diagram of another supply circuit known
in the art coupled to a load circuit;

[0033] FIG. 3 illustrates a block diagram of a supply circuit in
accordance with the present invention coupled to a total number of three
load circuits;

[0034]FIG. 4 illustrates a resonant current signal of the resonant
circuit;

[0035]FIG. 5 illustrates a schematic diagram of a device in accordance
with the present invention;

[0036]FIG. 6 illustrates a schematic diagram of an embodiment of a device
in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] FIG. 1 illustrates a block diagram of a supply circuit 1 known in
the art. The supply circuit 1 comprises a bridge circuit 3 and is coupled
to a power source 5. The power source 5 is a direct voltage source
Vin. The supply circuit 1 further comprises a resonant circuit 7 and
is coupled to a load circuit 9 comprising at least one loads, in FIG. 1 a
total number of four loads 11 forming together a load string 13. The
loads 11 may be LEDs, OLEDs or the like that are powered by the supply
circuit 1.

[0038] The bridge circuit 3 illustrated in FIG. 1 is based on a full
bridge comprising four switches M1, M2, M3 and M4,
which are exemplary MOSFETs in FIG. 1. The switching status of the
switches M1, M2, M3 and M4, i.e., whether they are
switched on or off, is controlled by a control unit not illustrated in
FIG. 1.

[0039] Furthermore, a snubber circuit 15 forms part of the bridge circuit
3, which comprises an inductance Lzvs and a capacitance Czvs.
The snubber circuit 15 provides an additional current Izvs, which is
required for discharging the MOSFET's (internal) capacitance in order to
achieve zero voltage switching (ZVS) leading to a reduction of switching
losses. Hence, the snubber circuit 15 serves for achieving a
high-efficiency and an additional ZVS, whereas ZVS is well-known in the
art and is therefore not explained in more detail hereinafter. The
snubber circuit 15 may also be integrated in a transformer together with
the resonant circuit 7. It should be noted that other measures may be
taken to achieve ZVS.

[0040] The supply circuit 1 and the load circuit 9 constitute a device 17,
which may be a consumer or a non-consumer product.

[0041] The resonant circuit 7 comprises an inductance Lres and a
capacitance Cres, which are connected in series in FIG. 1. The
inductance Lres and the capacitance Cres define a resonant
frequency and a resonant impedance of the resonant circuit 7. Hence, the
bridge circuit 3 and the resonant circuit 7 together form a series
resonant converter. The switches M1, M2, M3 and M4 of
the bridge circuit 3 are preferably switched pairwise with a duty-cycle
of 50%. It should be noted that other duty-cycles may be applied to the
switches such as 75% for the lower switches and for the upper switches
25%. Furthermore, the maximum switching frequency of the bridge circuit 3
is preferably half the resonant frequency of the resonant circuit 7.

[0042] In response to a direct current from the power source 5, the bridge
circuit 3 communicates a voltage signal to the resonant circuit 7 at a
switching frequency, which in turn communicates an alternating current
Ires to the load circuit 9.

[0043] Furthermore, decoupling diodes 19 are provided that may form part
of the resonant circuit 7 or the load circuit 9. Above all, a smoothing
capacitance Cout is connected in parallel to the load string 13,
which avoids a pulsating current in the loads 11. It should be noted that
the smoothing capacitance Cout is not mandatory and may be omitted.

[0044]FIG. 2 illustrates a block diagram of a further supply circuit 1'
known in the art and coupled to a load circuit 9'. As can be seen from
FIG. 2, the supply circuit 1' differs from the supply circuit 1
illustrated in FIG. 1 in that the bridge circuit 3' of the supply circuit
1' is based on a half bridge instead of a full bridge. Hence, the half
bridge only comprises two switches M1 and M2, which are
exemplary MOSFETs. Again, the driver to control the switches M1 and
M2 is not illustrated in FIG. 2.

[0045] In contrast to the supply circuit 1 of FIG. 1, a transformer 21
comprising a primary winding N1 and a secondary winding N2 is
provided in the supply circuit 1' of FIG. 2, which is coupled to a load
circuit 9' and the resonant circuit 7, the resonant circuit 7 again
comprising the inductance Lres and the capacitance Cres
defining a resonant frequency and a resonant impedance. The transformer
21 may serve to transform the input voltage Vin to a higher or a
lower output level Vout and is not necessarily provided in the
supply circuit 1'. Rather, it may be substituted by other additional
components such as a voltage doubler circuit or it may be simply omitted.

[0046] As can be seen from FIG. 2, the load circuit 9' comprises two
decoupling diodes 19 as well as two LED strings 13, which are arranged in
an anti-parallel configuration. Each of them is provided with a smoothing
capacitance Cout1 and Cout2. It will be appreciated that the
load configuration is variable and may vary depending on the application.

[0047] Both, the supply circuit 1 of FIG. 1 and the supply circuit 1' of
FIG. 2 show basic DSRC-I configurations that both provide a constant
average output current Iout without sensing the current. It shall be
noticed that the half-bridge configuration of the supply circuit 1' is
preferably used as less components are required and the control signals
are easier to handle than those of a full bridge.

[0048] It is obvious from FIG. 1 and FIG. 2 that the supply circuits 1 and
1' both comprise a resonant circuit 7, which is coupled to a load circuit
9, 9' comprising exemplary a plurality of loads 11, in particular load
strings 13. The loads 11 or the load strings 13 respectively may be LEDs
or OLEDs.

[0049] If a color is to be controlled for a certain consumer or
non-consumer application, it is advantageous if the supply circuit is
provided with at least three outputs, i.e., three load circuits 9, 9',
whereas each of the three load circuits may comprise at least one
different colored LED, e.g., one load circuit comprising at least one red
LED, another load circuit comprising at least one green LED and the third
load circuit comprising at least one blue LED. The different colored
light of the different load circuits may then be mixed in a certain ratio
to generate a desired color. Therefore, the at least one load 11 of a
load circuit 9 needs to be individually dimmed in order to perform color
control.

[0050] Up to now, dimming is commonly performed by varying the switching
frequency of the bridge circuit 3, 3', which brings along the
disadvantages described previously. In particular, individual full range
dimming of different outputs, i.e., of different load circuits 9, 9' is
not feasible with this method, as a change in switching frequency will
influence the current in all load circuits similarly. By contrast, the
present invention provides an individual full-range dimming of an
arbitrary number of load circuits 9, thus, allowing color control for a
large number of applications.

[0051] FIG. 3 illustrates a schematic diagram of a supply circuit 101
according to the present invention coupled to a total number of three
load circuits 109, 109' and 109''. Each of the load circuits 109, 109'
and 109'' in turn comprises a number of loads 111 that are combined in
two load strings 113 connected in an anti-parallel configuration.

[0052] The number of loads 111 per load circuit 109, 109' and 109'' as
well as the number of load circuits itself may vary. Thus, it is possible
to provide only one load circuit 109 comprising a single load 111, which
may be a LED or the like. However, the present invention is particularly
advantageous, if at least three load circuits 109, 109' and 109'' are
coupleable to the supply circuit 101.

[0053] It will be appreciated that also the configuration of the loads 111
may vary. In FIG. 3 they are merely exemplary configured in the form of
load strings 113, whereas two load strings 113 are provided per load
circuit 109, 109', 109'', which are above all configured in an
anti-parallel way, i.e., with opposite polarizations.

[0054] The supply circuit 101 comprises a bridge circuit 103, which is
based on a half bridge, whereas the half bridge and its advantages have
previously been addressed to in the context of FIG. 2. The supply circuit
101 further comprises at least one, in FIG. 3 a total number of three
resonant circuits 107, 107' and 107''. Each of the resonant circuits 107,
107' and 107'' in turn is coupleable to a load circuit 109, 109' and
109''.

[0055] The switches M1 and M2 of the bridge circuit 103 are
preferably controlled by means of a control unit, which is adapted to
switch the bridge circuit with a duty-cycle of 50% and a maximum
switching frequency, which is half the resonant frequency of the resonant
circuits 107, 107', 107'', whereas the resonant circuits consist of the
inductance Lres and the capacitance Cres. The dimensioning of
Lres and Cres may vary in each resonant circuit 107, 107' and
107''. In particular, the resonant frequency of one or more resonant
circuits may differ from those of the other resonant circuits. In this
case, the resonant circuit 107, 107' or 107'' having the lowest resonant
frequency determines the limit for the switching frequency of the bridge
circuit 103 in order to fulfill the condition that the maximum switching
frequency of the bridge circuit is half the resonant frequency of the
resonant circuit. In this case, the control unit is adapted for providing
a maximum switching frequency of the bridge circuit 103, which is half of
the lowest resonant frequency of a plurality of resonant circuits 107,
107', 107''.

[0056] For an individual dimming of loads 111 in accordance with the
present invention, the supply circuit 101 comprises at least one supply
switching unit 129 to connect or disconnect the one or more loads 111
from the supply circuit 101. The supply switching unit 129 is coupled
between the bridge circuit 103 and an associated load circuit 109 for
connecting and disconnecting the load circuit 109 from the bridge circuit
103. A control unit 131 for driving the supply switching unit 129 is
adapted for switching on or off the supply switching unit 129 in
synchronization with a resonant current Ires of the resonant circuit
107, 107', 107'' associated with said supply switching unit 129, which
will be explained in detail hereinafter.

[0057] Moreover, the at least one supply switching unit 129 is preferably
a MOSFET. However, it should be noted that the supply switching unit 129
may also comprise any other component being suited for connecting or
disconnecting the one or more loads 111 and the load circuit 109
respectively from the bridge circuit 103. The supply switching unit 129
may also comprise a plurality of components.

[0058] It is obvious from FIG. 3 that in particular, the resonant circuit
107, 107', 107'' is provided with at least one supply switching unit 129.
Preferably, the number of resonant circuits corresponds to the number of
supply switching units 129. It should be noted that the at least one
supply switching unit 129 is connected in series to the one or more loads
111 of the load circuit 109, 109', 109'' or in other words, the supply
switching unit 129 is interconnected between the bridge circuit 103 and
the respective load circuit 107, 107' or 107''. Hence, a series
connection of the resonant inductance Lres, the resonant capacitance
Cres, the entity of loads 111 and the supply switching unit 129 is
resulting.

[0059] In order to use the supply circuit 101 for various applications,
preferably a plurality of resonant circuits 107, 107' and 107'' is
coupled to the bridge circuit 103 and each resonant circuit 107, 107' and
107'' in turn being coupleable to at least one load circuit 109, 109',
109''. The respective load circuit 109, 109', 109'' coupleable to a
resonant circuit 107, 107', and 107'' may comprise one or more LEDs
and/or one or more OLEDs.

[0060] It is obvious from FIG. 3 that a device 117 according to the
invention comprises the supply circuit 101, that is the bridge circuit
103, the resonant circuits 107, 107', 107'', the supply switching units
129 and the load circuits 109, 109', 109''.

[0061] A preferred embodiment of the present invention, which is
illustrated in FIG. 3 incorporates the following constitution: The supply
circuit 101 comprises three resonant circuits 107, 107', 107'', whereas
the first of said resonant circuits 107 is coupled to a load circuit 109
comprising at least one red LED and/or OLED, the second of said resonant
circuits 107' is coupled to a load circuit 109' comprising at least one
green LED and/or OLED, and the third of said resonant circuits 107'' is
coupled to a load circuit 109'' comprising at least one blue LED and/or
OLED.

[0062] If the loads 111 are LEDs or OLEDs, a variable light output of
different colored (O)LEDs can then be realized. As mentioned above, the
supply switching units 129 are exemplary MOSFETs, however, any other
suited type of switching device may be applied.

[0063] The supply switching units 129 are preferably controlled by the
control unit 131, which may be additionally adapted to control the
switches M1 and M2 of the bridge circuit 103. In this case,
only one single control unit is required for both, controlling the bridge
circuit 103 as well as the supply switching unit 129.

[0064] As can be seen from FIG. 3, the resonant circuits 107, 107' and
107'' are each provided with a supply switching unit 129. The supply
switching unit 129 of resonant circuit 107 is denoted in FIG. 3 as
MR, supply switching unit 129 of resonant circuit 107' is denoted as
MG and supply switching unit 129 of resonant circuit 107'' is
denoted as MB. The denotation indicates that the loads 111 of load
circuit 109 are preferably red LEDs, the loads 111 of load circuit 109'
are preferably green LEDs and the loads 111 of load circuit 109'' are
preferably blue LEDs.

[0065] Each supply switching unit 129 is connected in series to the
respective loads 111 or is coupled between the bridge circuit 103 and the
associated load circuit 109. In the following, the operation of the
supply switching unit 129 will be explained in more detail. It will be
appreciated that employment of the term "a load 111" is only used as a
generic term that may include any number of loads as well as different
types of loads such as LEDs and/or OLEDs, whereas the configuration of
the loads is in addition arbitrary.

[0066] In order to control a load 111 of a load circuit 109, 109' or
109'', the supply switching unit 129 is switched on or off by means of
the control unit 131. If the load 111, in particular one or more LEDs is
to be disconnected, e.g., in order to reduce the light output, the supply
switching unit 129 is switched off preferably during the second negative
half-wave of the resonant current Ires, thus, during the time period
t0 illustrated in FIG. 4, whereas FIG. 4 shows the basic, not
interrupted resonant current Ires depending on a change in time t.
Additionally, the periodic time Tswitch of the bridge circuit 103 is
illustrated in FIG. 4. It will be appreciated that although the resonant
current Ires is AC, there is no bidirectional blocking switch
required.

[0067] After switching off the supply switching unit 129, i.e. the MOSFET,
the resonant current Ires commutates on the not illustrated
intrinsic body diode of the MOSFET and stops after it reaches zero at the
time tx illustrated in FIG. 4. Further current flow does not occur,
because the resonant capacitance Cres is charged the way that the
body diode of the MOSFET blocks further current flow. Hence, current flow
from the load circuit 109 back to the bridge circuit 103 is prevented by
the supply switching unit 129. In this condition, the load 111 is
disconnected from the supply circuit 101.

[0068] Outgoing from the situation that a load 111 has been switched off,
i.e. disconnected from the supply circuit 101 by switching off the
respective supply switching unit 129, the load 111 is switched on again
by switching on the respective supply switching unit 129, whereas
switching on is preferably performed in the same time interval t0 as
switching off the supply switching unit 129, namely during the second
negative half-wave of the resonant current Ires. As a consequence,
the resonant current Ires will restart with the second positive
half-wave after the time tx as if no interruption had occurred, in
particular, if a full bridge is applied.

[0069] If a half bridge is applied as illustrated in FIG. 3, the second
positive half-wave of the resonant current Ires preferably restarts
directly after switching on the supply switching unit 129 in the time
interval t0 and not only at the time tx. Hence, if using a half
bridge, the second positive half-wave of the resonant current Ires
may begin before the time tx. However, this won't cause any problems
as after the expiration of the second positive half-wave the current will
stay zero for a certain period of time, in particular if a sufficiently
high output voltage is applied.

[0070] In order to reduce switching losses, the supply switching unit 129
is preferably switched as late as possible in the time interval t0,
i.e., as close as possible to the time tx. Above all, this will
solve the problem of switching the supply switching unit 129 prior to the
time tx in case that a half bridge is applied and hence the
subsequent second positive half-wave of the resonant current Ires
already starting before the time tx.

[0071] Hence, the light output of a load circuit 109, 109' and 109'' may
be varied only by varying the number of omitted switching cycles
Tswitch of the bridge circuit 103.

[0072] The information about when the resonant current Ire, is in the
second negative half-wave of the switching period or switching cycle
Tswitch, i.e., in the time interval t0 for switching the supply
switching unit 129 is gained from the switching signals of the bridge
circuit 103. The relation is very simple because the behavior of the
resonant current Ires is known. Hence, the switching operation of
the supply switching units 129 is adjusted to the switching signals of
the half or full bridge circuit 103.

[0073] The switching cycle of the bridge circuit 103, i.e., of switches
M1 and M2 is not interrupted while switching on and off the
supply switching unit 129. In addition, the bridge circuit 103 operates
constantly with complete zero voltage switching because it is not
interrupted or influenced by the dimming of the individual loads 111.

[0074] It shall be understood that each supply switching unit 129, i.e.,
MR, MG and MB may be switched on and off fully independent
from each other. Also the number of switching cycles Tswitch, during
which a supply switching unit 129 is switched on or off is variable and
independent from each other. Consequently, each load circuit 109, 109',
109'' being coupled to a supply switching unit 129 may be individually
controlled only by switching on or off the respective supply switching
unit 129 for a desired number of switching cycles of the bridge circuit
103.

[0075] It is obvious that a large number of supply switching units 129 may
be provided, each realizing an individual full-range dimming of one or
more LEDs, OLEDs or the like. Thereby, only one central converter, i.e.,
one central bridge circuit 103 is required.

[0076] It will be appreciated that according to the present invention, the
load is not bypassed by means of a supply switching unit connected in
parallel to a load 111, which has harmful influences on the converter
stability. According to the present invention, the supply switching unit
129 is rather connected in series to the load 111 in order to avoid
bypassing the load. Instead, the load is disconnected for a desired
number of switching cycles Tswitch of the bridge circuit 103, as the
supply switching unit 129 blocks a current flow back to the bridge
circuit 103. Meanwhile, the supply circuit 101 is not at all influenced
by the supply switching unit 129 as the load 111 is not bypassed, but
current flow is interrupted.

[0077] Consequently, this invention offers the advantages of an individual
full-range dimming down to zero, whereas the complete ZVS operation is
kept also during dimming of the individual loads 111. Another advantage
of the present invention is low additional losses as MOSFETs with a low
RDSon can be used. Furthermore, the supply circuit topology can be
enhanced to any desired number of load circuits 109 and loads 111 and
with this to any desired number of individual dimmable LEDs. Finally,
interleaving of LEDs is also possible with the present invention to
reduce the overall input current ripple.

[0078] As mentioned above, the DSRC-I preferably used for the present
invention may instead of a half-bridge comprise a full-bridge
configuration. However, the major advantage of the half-bridge is that
the source of the supply switching unit 129, i.e., the source of the
MOSFET is connected to ground. Consequently, driving this MOSFET is very
easy. Furthermore, fewer components are required as can be seen from FIG.
4: The device 117 comprising the supply circuit 101 and the three load
circuits 109, 109' and 109'' only requires three supply switching units
129 and two switches M1 and M2 for realizing an individual
dimming functionality for each load circuit 107.

[0079]FIG. 5 illustrates a schematic diagram of a device 117 in
accordance with the present invention comprising the load circuits 109,
109' and 109'' and the supply circuit 101 comprising the bridge circuit
103, the resonant circuits 107, 107', 107'' and the supply switching unit
129. It becomes obvious that for the individual dimming of different
colored LEDs only one central converter unit is required. The individual
full-range dimmer interposed between the load circuits 109, 109' and
109'' and the bridge circuit 103 comprises the resonant circuits 107,
107' and 107'' with the associated supply switching units 129. It is
further obvious from FIG. 5 that the control unit 131 is adapted for
driving the supply switching units 129 and the bridge circuit 103 at the
same time.

[0080] As the light output of an LED is basically proportional to the
current, the supply circuit 101 can generate all desired colors without
the need of any feedback loop, because of the constant average current
output behavior of the DSRC-I. However, changes in temperature or aging
of the LED can influence the light to current ratio of the LED. If the
light output has to be more precise, the light of each LED can be sensed
by a photosensitive component such as a photo sensor, in particular a
photodiode. The light output can then be controlled simply by adjusting
the corresponding load current. Sensing the light output is advantageous
as it provides lower losses and more advantages than sensing the load
current.

[0081] For example, it allows the easy detection of a system fault or a
damaged LED as well as calibrating the LEDs or compensating aging
effects. Above all, a perfect color control is feasible with the photo
sensitive component.

[0082]FIG. 6 illustrates a schematic diagram of a device 117' in
accordance with the present invention comprising photosensitive
components 133, 133', 133''. Preferably, at least one photosensitive
component 133, 133', 133'' is assigned to at least one load circuit 109,
109', 109'', in particular to the loads 111 of the load circuits. It is
conceivable that each load 111 is associated with a photosensitive
component 133. Those areas, where the currents in the device 117' are
particularly known are denoted in FIG. 6 with 135, 135' and 135''. These
currents may be adapted particularly by means of the control unit 131 in
accordance with the sensing result of the photosensitive component 133,
whereas the sensing results of the photosensitive components 133, 133'
and 133'' are fed to the control unit 131 as an example by
interconnections 137, 137' and 137''.

[0083] While the invention has been illustrated and described in detail in
the drawings and foregoing description, such illustration and description
are to be considered illustrative or exemplary and not restrictive; the
invention is not limited to the disclosed embodiment. Other variations to
the disclosed embodiment can be understood and effected by those skilled
in the art in practicing the claimed invention, from a study of the
drawings, the disclosure, and the appended claims.

[0084] In the claims, the word "comprising" does not exclude other
elements or steps, and the indefinite article "a" or "an" does not
exclude a plurality. A single element or other unit may fulfill the
functions of several items recited in the claims. The mere fact that
certain measures are recited in mutually different dependent claims does
not indicate that a combination of these measured cannot be used to
advantage.

[0085] Any reference signs in the claims should not be construed as
limiting the scope.